creating attic/ for cleaning up old code; moving data_graph.py into it
Simplifying use of ProtocolHelper

Removing references to app.ph, self.ph, etc. and just directly calling functions in protocol.py
Getting rid of one more ProtocolHelper reference
Adding session.py and placeholders for some types of sessions
Moving bokeh_ggplot and plot_expr_graph into attic/ for now

They are using an older approach which we can revamp later to use the new glyph-based python interface
Making plot server a little more verbose about its ports
Adding objects.py, for PlotObject and related subclasses

Initial check-in, work in progress